tips/tricks for tapping a hole?

A couple of my crossmember mounting bolts were stripped out so I'm drilling and tapping them to 1/2"-13. I drilled the first one to 3/8" and am using a taper tap but can't seem to get it to start. I was using WD40 as a light lube.
Any tips or tricks I can use to keep the tap square to the hole while trying to turn it and get it started?
TIA

When doing it by hand, I use one hand to put pressure on the tap and the other to turn it slowly. The key part is keeping the tap square (as youve pointed out) Sometimes it just takes a little while to finally start it.

I have also used a drill which seems to be easier but there is more risk of breaking your tap off in the hole that way. Make sure you lube it up and go very slowly with the drill. Keep the drill square and apply pressure and the tap should catch somewhat easily. Also be sure to back it out and clean the tap and the hole every little bit. Should keep it from binding and breaking.

HTH

First problem - 1/2"-13 requires a tap drill of 27/64". You try to run a 1/2"-13 tap into a 3/8" hole, you're going to break the thing (I know it's only 3/64" difference. It damn sure does matter!)

Second - use a taper tap - with 3-5 threads ground to a taper at the sharp end. This will be easier to start (you can follow up with a bottom tap if you need a blind hole tapped all the way down.)

A tap will "follow" the hole it's being run into, so just don't try to run it down at an angle. Half to three-quarter turn in, quarter-turn back to break the chips. I find it easiest to use grease as a lube, and you should back it all the way out every half-dozen turns or so to clean it (more often if you're going into aluminum, the chip clogs up the flutes quickly.)

john is very right. the 3/64 measures out to .043... In perspective, a soda can is .004 thick, so about 11 times the thickness of a soda can is how much too small the diameter of your hole will be... it's significant.

the taper tap will help get it started too, but the biggest thing is to go slow and wait for it to bite. once it bites you'll be fine.... but getting the initial bite is what i have found to be the hardest part. bottom taps are nice for cleaning rusty threads or something like that, but they can make life miserable for starting tapping a hole from scratch.

Since you don't have a drill chart, here is a very easy formula for determining the proper drill size...

Major Diameter - Pitch = Drill Size


Major Diameter is equal to the nominal outside diameter of the bolt.
IE: 1/2-13 = .5
3/8-16 = .375
1/4-20 = .25
etc.

Pitch is equal to 1/# threads per inch.
IE: 1/2-13 = .077
3/8-16 = .063
1/4-20 = .05
etc.


Let's take for example 1/4-20,

1/4 = .25
1/20 = .05
.25-.05 = .2

A #7 drill is .201.... In the machining world, we call that close enough!

Hope that helps.

Too big for the hole - STORY OF MY LIFE:exclamati

Seems obvious now. I'm using a taper tap, and will switch from WD40 to grease, after I get the right size drill bit. Thanx, guys. :cheers:

Original_MudButt said:

Major Diameter - Pitch = Drill Size

Click to expand...

...soo that's how they do it. :confused1

but wouldn't it make more sense to drill to the minor diameter of the thread? or does this also work out to that as well?

I ran the proper 27/64 drill through there and then the tap went in slick as anything. Thanx again, guys. atriot:

asp387 said:

...soo that's how they do it. :confused1

but wouldn't it make more sense to drill to the minor diameter of the thread? or does this also work out to that as well?

Click to expand...

It probably would, but most common tap drill charts specify 75% thread engagement so you have operating clearance for the mating parts to attach (threads aren't supposed to have any wedging action until the head bottoms out on the upper surface of whatever.)